In certain industrial settings, the presence of organic fluids that are combustible gases, such as alkanes, like methane or hydrogen, may reflect leakages in piping systems, would constitute a fire and explosion hazard if permitted to accumulate undetected. Another high-risk environment is methane in the air of underground mine shafts. Precautions are needed to protect from such hazards by continuously or sporadically monitoring flowing fluid streams to preclude hydrocarbon concentrations from reaching a dangerous level.
In other commercial applications, there is a need to monitor the exhaust solvent (organic) fluids from such sources as paint and varnish applications, chemical processes and internal combustion engine exhausts, all of which may be expended to the atmosphere. Moreover, recent governmental action through more stringent air pollution control laws, are intended to limit the admission of organics, and perhaps carcinogenic organic chemicals into the atmosphere. The growing need for detection and remedial steps is thus well documented.
Available organic fluid detectors may conventionally utilize a detector filament or catalytic detector elements, elements which may be electrically heated to provide for oxidation at the filament surface of the organic components. Such catalytic elements may be included in a detection circuit, such as a Wheatstone bridge circuit, containing a reference element which does not provide for a reaction of the organic fluid at the reference element surface. The resistance of such a catalytic element is a function of the temperature of the element, and when the filament resistance is increased by the heat of reaction of the organic fluid, the presence of the organic fluid may be detected by its differential effect on the catalytic element in respect to the reference element. There has been a long standing need for providing a low cost, high sensitivity organic fluid concentration detector, typically for use around organic fluid storage facilities.
The prior art methods work for high concentrations of organic fluids, but when it becomes necessary to strain the sensitivity of such devices to detect low concentrations (ranging from five percent downwardly), the prior art methods, evidence a lack of stability. This may apparently be caused by electrical noise in the detector, and long-term drifting of the fluid concentration indicator. This may also be caused by short-term electrical noise and long-term resistance changes in the detector element.
Early attempts were made to improve stability by adding a compensating filament. A similar filament to that of the fluid detector filament was placed on an adjacent arm of a Wheatstone bridge circuit. Changes in ambient temperature and power supply voltage produce similar changes in the two filaments, but their location in the circuit is such as to produce oppositely acting responses in the output signal. The circuit is therefore stabilized against changes that affect the two filaments equally. Oxidation of the organic fluid sample to be analyzed is prevented by not exposing the compensator filament to the organic fluid sample or by operating it at a lower temperature. Instruments of this type are described in U.S. Patents to R. E. Hartline, Combustible Gas Indicator, 2,279,397; and M. G. Jacobson, Combustible Gas Indicator, 2,244,366. These detection schemes work well when the organic fluid concentration is high. (5% methane in air, or more, full scale). A critical deficiency is that their stability decreased as the full scale sensitivity increased.
In the more recent past, attempts have been made to solve this chronic shortfall. For example, Hartline invented an organic fluid detector system based on the use of thermal hysteresis (U.S. Pat. No. 2,617,716). While he was successful in detecting high fluid concentrations with great zero stability, the device failed to have the necessary sensitivity to detect low levels of organic fluids.
Systems have been proposed for keeping of the sensing element at ependable levels in the face of widely varying organic fluid concentrations. However, they have practical disadvantages, so improved detectors for low organic fluid detection and measurement are still to be sought.